The present invention relates generally to the field of conductive polymer positive temperature coefficient (PTC) devices. More specifically, it relates to conductive polymer PTC devices that are of laminar construction, with more than a single layer of conductive polymer PTC material, and that are especially configured for surface-mount installations.
Electronic devices that include an element made from a conductive polymer have become increasingly popular, being used in a variety of applications. They have achieved widespread usage, for example, in overcurrent protection and self-regulating heater applications, in which a polymeric material having a positive temperature coefficient of resistance is employed. Examples of positive temperature coefficient (PTC) polymeric materials, and of devices incorporating such materials, are disclosed in the following U.S. Pat. Nos.:
3,823,217--Kampe PA0 4,237,441--van Konynenburg PA0 4,238,812--Middleman et al. PA0 4,317,027--Middleman et al. PA0 4,329,726--Middleman et al. PA0 4,413,301--Middleman et al. PA0 4,426,633--Taylor PA0 4,445,026--Walker PA0 4,481,498--McTavish et al. PA0 4,545,926--Fouts, Jr. et al. PA0 4,639,818--Cherian PA0 4,647,894--Ratell PA0 4,647,896--Ratell PA0 4,685,025--Carlomagno PA0 4,774,024--Deep et al. PA0 4,689,475--Klieiner et al. PA0 4,732,701--Nishii et al. PA0 4,769,901--Nagahori PA0 4,787,135--Nagahori PA0 4,800,253--Kleiner et al. PA0 4,849,133--Yoshida et al. PA0 4,876,439--Nagahori PA0 4,884,163--Deep et al. PA0 4,907,340--Fang et al. PA0 4,951,382--Jacobs et al. PA0 4,951,384--Jacobs et al. PA0 4,955,267--Jacobs et al. PA0 4,980,541--Shafe et al. PA0 5,049,850--Evans PA0 5,140,297--Jacobs et al. PA0 5,171,774--Ueno et al. PA0 5,174,924--Yamada et al. PA0 5,178,797--Evans PA0 5,181,006--Shafe et al. PA0 5,190,697--Ohkita et al. PA0 5,195,013--Jacobs et al. PA0 5,227,946--Jacobs et al. PA0 5,241,741--Sugaya PA0 5,250,228--Baigrie et al. PA0 5,280,263--Sugaya PA0 5,358,793--Hanada et al.
One common type of construction for conductive polymer PTC devices is that which may be described as a laminated structure. Laminated conductive polymer PTC devices typically comprise a single layer of conductive polymer material sandwiched between a pair of metallic electrodes, the latter preferably being a highly-conductive, thin metal foil. See, for example, U.S. Pat. Nos. 4,426,633--Taylor; 5,089,801--Chan et al.; 4,937,551--Plasko; and 4,787,135--Nagahori; and International Publication No. WO97/06660.
A relatively recent development in this technology is the multilayer laminated device, in which two or more layers of conductive polymer material are separated by alternating metallic electrode layers (typically metal foil), with the outermost layers likewise being metal electrodes. The result is a device comprising two or more parallel-connected conductive polymer PTC devices in a single package. The advantages of this multilayer construction are reduced surface area ("footprint") taken by the device on a circuit board, and a higher current-carrying capacity, as compared with single layer devices.
In meeting a demand for higher component density on circuit boards, the trend in the industry has been toward increasing use of surface mount components as a space-saving measure. Surface mount conductive polymer PTC devices heretofore available have been generally limited to hold currents below about 2.5 amps for packages with a board footprint that generally measures about 9.5 mm by about 6.7 mm. Recently, devices with a footprint of about 4.7 mm by about 3.4 mm, with a hold current of about 1.1 amps, have become available. Still, this footprint is considered relatively large by current surface mount technology (SMT) standards.
The major limiting factors in the design of very small SMT conductive polymer PTC devices are the limited surface area and the lower limits on the resistivity that can be achieved by loading the polymer material with a conductive filler (typically carbon black). The fabrication of useful devices with a volume resistivity of less than about 0.2 ohm-cm has not been practical. First, there are difficulties inherent in the fabrication process when dealing with such low volume resistivities. Second, devices with such a low volume resistivity do not exhibit a large PTC effect, and thus are not very useful as circuit protection devices.
The steady state heat transfer equation for a conductive polymer PTC device may be given as: EQU 0=[I.sup.2 R(f(T.sub.d))]-[U(T.sub.d -T.sub.a)], (1)
where I is the steady state current passing through the device; R(f(T.sub.d)) is the resistance of the device, as a function of its temperature and its characteristic "resistance/temperature function" or "R/T curve"; U is the effective heat transfer coefficient of the device; T.sub.d is temperature of the device; and T.sub.a is the ambient temperature.
The "hold current" for such a device may be defined as the value of I necessary to trip the device from a low resistance state to a high resistance state. For a given device, where U is fixed, the only way to increase the hold current is to reduce the value of R.
The governing equation for the resistance of any resistive device can be stated as EQU R=.rho.L/A, (2)
where .rho. is the volume resistivity of the resistive material in ohm-cm, L is the current flow path length through the device in cm, and A is the effective cross-sectional area of the current path in cm.sup.2.
Thus, the value of R can be reduced either by reducing the volume resistivity .rho., or by increasing the cross-sectional area A of the device.
The value of the volume resistivity .rho. can be decreased by increasing the proportion of the conductive filler loaded into the polymer. The practical limitations of doing this, however, are noted above.
A more practical approach to reducing the resistance value R is to increase the cross-sectional area A of the device. Besides being relatively easy to implement (from both a process standpoint and from the standpoint of producing a device with useful PTC characteristics), this method has an additional benefit: In general, as the area of the device increases, the value of the heat transfer coefficient also increases, thereby further increasing the value of the hold current.
In SMT applications, however, it is necessary to minimize the effective surface area or footprint of the device. This puts a severe constraint on the effective cross-sectional area of the PTC element in device. Thus, for a device of any given footprint, there is an inherent limitation in the maximum hold current value that can be achieved. Viewed another way, decreasing the footprint can be practically achieved only by reducing the hold current value.
There has thus been a long-felt, but as yet unmet, need for very small footprint SMT conductive polymer PTC devices that achieve relatively high hold currents.